1. Field of the Invention
The present invention relates generally to a power tool, and more particularly to an impact drill having an impact mechanism for delivering a rapid succession of axial blows to a rotating drill bit.
2. Prior Art
Various impact drills are known in which an impact mechanism includes a motion translation means such as a crank mechanism for translating a rotary motion of an electric motor to a linear reciprocating motion of a piston. The piston is connected to a crank arm via a connecting rod and reciprocates in a cylinder to produce compressed air which in turn works a striker to move to and fro to strike the upper end of a rotating drill bit.
The known impact drills of the foregoing construction involve a great mechanical loss partly because various seals such as O-rings must be provided to seal the compressed air against leakage from the interface between movable members such as the piston and the striker, and stationary members such as the cylinder and a striker housing. A further mechanical loss is produced when the rotary motion is translated into the linear reciprocating motion. In a rotational power transmitting system of the known impact drills, the cylinder and the piston and other parts disposed in the cylinder are rotated in unison to transmit the rotary motion to the drill bit. This arrangement requires a large-sized bearing for the cylinder and hence the rotational power transmitting system is large in size and heavy in weight. In order to accommodate the great mechanical loss, it is necessary to increase the motor size which however results in an enlargement of the overall size of the impact drill. Further, the impact drill having the pneumatic impact power transmitting system is relatively long and hence uneasy to handle.